1. Field of the Invention
This invention relates to improvement of the method of cooling reflector lamps using a high brightness light source.
2. Prior Art
The tungsten-halogen family of incandescent lamps can be used for many light projection purposes. The present invention relates to reflector lamps using the tungsten-halogen incandescent lamps. The bulbs used in these lamps are normally single ended, compact and made of quartz or high silica contend glass. This material is used so that the bulb will withstand resultant high pressures and high temperatures. The quartz bulb also serves to contain a small volume of hot gas about the filament to aid in the performance of the lamp.
The reflector lamp normally uses one ofthe tungsten-halogen incandescent lamps in conjunction with a reflecting surface. This reflecting surface is permanently cemented to the lamp near the seal end of the bulb. Reflectors are normally made of glass in order to be thermally compatable with the lamp and because of the reflective qualities that may be imparted to such glass.
These new tungsten-halogen reflector lamps supply a very compact source of intense light, and have many applications. They are specifically useful in light projection applications such as photographic film, projectors and photographic enlargers.
The benefits provided by this compact source of light have often been greatly reduced because of the intense heat that the lamp produces and the inability to deal with this heat. This intense heat causes several kinds of problems that must be overcome in order to utilize the compact lamp in a reflector type operation. Tungsten halogen lamps of 300 watts have a temperature of 600.degree. centigrade or more, as contrasted with inert gas incandescent lamps which have a substantially lower temperature.
One problem occurs because the two kinds of glass used in the lamp and reflector have different thermal expansion rates. When the joint between the reflector, made from a fused silica glass, and the bulb, made of quartz, is subjected to heat, the two types of glass expand at different rates causing the joint to break or crack. Often the joint between the fused silica reflector and the quartz bulb is of a permanent rigid connection. This inflexibility causes the joint between the bulb and reflector to crack as the reflector expands at a different rate than the quartz bulb. Therefore, in normal operation the maximum temperature must be controlled and limited within a specific range of temperatures, or failure of the bulb or reflector will occur.
Another problem involves the breaking of the seal in the glass between the filament leads of the quartz bulb. The quartz bulb will normally take severe temperature changes, however, the filament leads extend through the end of the bulb where the glass area becomes small in relation to the size of the filament leads. This area of glass when subjected to very high temperatures, will crack due to the difference in thermal coefficients of expansion between the filament leads and the quartz bulb. When the seal of the bulb is broken the filament burns.
Still another problem occurs because the reflector is permanently fastened to the open ends of the quartz bulb. The problem arises because the reflector further reduces cooling by decreasing the area available for dissipating heat and increases the sensitivity of the seal which increase the probability of the seal cracking.
The tungsten-halogen reflector lamps are often used in apparatus that require a very low light leakage since in projectors or photographic enlargers leakage of light impedes the utility of the use. Therefore, baffles often have to be placed to restrict the passage of light from the compact housing surrounding the lamp. These baffles increase the bulk size of the housing rather than decrease it and partially eliminate any benefits derived from the compact light source.
There have been several attempts to solve the problems created by the extreme heat of the tungsten-halogen lamps. One method, and probably the most widely used and suggested by most manufacturers is to direct a blast of air at the seal of the quartz bulb and at the joint between the quartz bulb and the fused silica glass reflector. This approach will solve the problem and will control the extreme heat generated if the air stream is sufficient. However, this solution is not always practical because, in order to direct the air stream a fan with blower ducts must be installed near the housing of the lamp source. Thus, the benefits gained by utilizing the compact high brightness light source is again negated because the size of the apparatus must be increased to be able to contain the blower and the blower ducts.
The forced air can cool the seal, reflector and joint from the rear of the lamp. The open end of the reflector containing the bulb and filament does not need forced air cooling and operates with normal lamp life at a temperature set by radiation cooling only. However, this radiation must still be considered in the overall heating analysis.
Still another problem occurs when a reflector light is used in an apparatus that requires low light leakage since baffles have to be installed to conduct and restrict the light. Here again, the value of the compact size of the intense light source is lost because of the added baffles. Thus, in prior art, in order to use the efficient compact light source an awkward, bulky apparatus must be used in order to house the cooling and shielding equipment that reduces both the light leakage, and the amount of heat created by the tungsten-halogen lamp. This not only results in awkward and bulky housings but also increases expense. In addition, the blower that cools the rear seal is often noisy and reduces the desirability of using the tungsten-halogen lamp.